Supplements are popular for people wishing to lose weight and build muscle, but some are dangerous.

Millions of people take sports supplements hoping for a range of health benefits, from weight loss to muscle building. But some supplements are being sold illegally and can be very harmful.

Sports supplements have become increasingly popular among gym-goers. People who are interested in fitness and improving their physique may opt for supplements that can enhance their muscle growth when combined with exercise, such as weightlifting. They may also look for ways to control their appetite when they're trying to lose weight, as part of a bodybuilding diet.

There is a growing industry in sports nutrition supplements available on the high street and online. Illegal supplements, including some claiming to be "fat burning" or "slimming", have been linked to a small number of deaths. Despite being illegal to sell, there is evidence that these are still available to buy online, mainly from suppliers based outside the UK.

It's important to be aware that products sold from a website or supplier based outside the UK or Europe may not pass the same safety standards as those within Europe.

Building muscle through protein

Protein is an important part of our diet and is key to building and maintaining all types of body tissue, including muscle. It contains amino acids, the building blocks used for muscle growth.

Protein powders, available as shakes, bars and capsules, are one of the most popular muscle-building supplements.

They are legally available to buy over-the-counter as well as online. They are marketed as helping to promote your body's muscle growth, aid metabolism (helping with weight loss), help you reach peak physical performance, boost energy and fight the ageing process.

"Users may choose to take them before, during and after training to enhance performance and improve recovery, add them to meals to boost their protein or drink them between meals as a high-protein snack," says Azmina Govindji from the British Dietetic Association (BDA).

"But they could get the same benefits from introducing high-protein foods to their diet as snacks or adding them to their normal meals to enhance the protein content. Although protein shakes are convenient, not all of them are suitable to be used as a meal replacement, because they don't have all the vitamins and nutrients that a balanced meal would contain."

This means that bodybuilders who turn to protein supplements, instead of simply eating protein-rich foods, could be wasting their money.

There is also evidence that, in the long term, consuming too much protein can lead to an increased risk of osteoporosis and can also worsen existing kidney problems. The Department of Health advises adults to avoid consuming more than twice the recommended daily intake of protein (55.5g for men and 45g for women).

Advice for gym-goers taking protein supplements

Rick Miller, clinical and sports dietitian from the BDA, has the following advice for gym-goers and bodybuilders who want to take protein supplements:

"A simple change in foods (such as Greek yoghurt in the morning with muesli and fruit, rather than plain breakfast cereal and milk) will help enhance the protein content of a meal. After you've taken this step, fill in the gaps with a reputable brand of protein supplement. Always read the label carefully, take the recommended serving size and don't be tempted to take far more than is necessary, as this is not supported by the current evidence.

"If you're unsure, ask your GP to refer you to a registered dietitian for advice. Protein supplements are not recommended for children due to the lack of research into long-term effects."

Chris Gibbons, a competitive powerlifter from Chesterfield, says there is a danger that people may mistakenly view supplements as a quick fix to achieve their goals.

"There is a tendency to think that there is a magic powder or supplement that will give you the physique of your dreams, but there is no substitute for hard work and commitment," he says.

"Building strength takes years, not weeks or months. It is an act of discipline and must be earned through commitment to hard training and a good diet."

If you're worried or you have experienced side effects after taking any supplements, especially any that you have bought online, make an appointment to see your GP.

Illegal bodybuilding and sports supplements

UK drug regulator the Medicines and Healthcare products Regulatory Agency (MHRA) has warned people to be wary of buying illegal sports supplements, because they might contain dangerous ingredients that could cause kidney failure, seizures and heart problems.

An MHRA investigation found that 84 illegal products, such as energy and "muscle-gain" products, were being sold containing dangerous ingredients such as steroids, stimulants and hormones. Among products that were taken off the market was a steroid product called Celtic Dragon. This product left two men hospitalised with severe jaundice and liver damage.

David Carter, the MHRA's manager of the borderline medicines section, says: "People need to be aware that buying illegal sports supplements can seriously damage your health. The products may claim to boost your energy or muscle but they could contain unapproved ingredients that can cause kidney failure, heart problems or seizures."

Even legal, over-the-counter supplements can cause you harm. For example, if you are taking any medicines as well as the supplements, the supplements could stop the medicine from working properly. Always read the label and, if in doubt, talk to your pharmacist.

In addition, many health claims made about products, foods and medicines sold online are not proven.

Risks of steroid use

Although available with a doctor's prescription for a variety of clinical reasons, some steroids are misused when taken as performance-enhancing drugs. They are attractive because they are based on the male hormone testosterone and can therefore improve endurance and performance, and stimulate muscle growth.

"But they can also enhance aggression," warns Rick Miller, in reference to what is commonly called "roid rage".

"Other major effects of steroid use include increase in blood pressure, direct kidney and heart damage, liver damage, acne and sexual promiscuity," he says.

Weight loss drug DNP linked to deaths

One product being sold illegally but still available online, mainly from suppliers based outside the UK, is 2,4-Dinitrophenol (DNP). DNP is an industrial chemical that isn't fit for human consumption. It is highly toxic and causes significant side effects, and has led to at least three reported deaths.

DNP is thought to be particularly popular among bodybuilders, who are attracted to its promises of quick-fix rapid weight loss.

Other names for DNP, which comes in a pill or powder form, include:

Dinosan

Dnoc

Solfo Black

Nitrophen

Aldifen

Chemox

DNP is dangerous because it speeds up the metabolism too quickly. This can trigger harmful side effects, such as:

restlessness

flushed skin

yellow skin

headaches

having an unusually fast heartbeat

breathing too fast

dizziness

sweating a lot

feeling very thirsty (dehydration)

feeling sick (nausea)

fever

vomiting

Using DNP over a long period of time can lead to cataracts and peeling skin, and may cause damage to the heart and nervous system. Animal studies have suggested that DNP may also cause cancer and increase the risk of birth defects.

If you think you have taken DNP, you should seek medical advice immediately.

The Food Standards Agency (FSA) is working with the police and local authorities to prevent DNP being sold illegally to the public, particularly online.

However, many websites offering DNP for sale are based abroad, making this work harder. Some of these sites offer illegal products alongside legal weight-loss drugs, making it very difficult to tell the difference.